Friday, July 3, 2009

COSA: A New Kind of Programming, Part V

In Part IV, I described the overall control architecture used in the COSA programming model and what happens to a component when it receives a ‘Start’ or ‘Stop’ signal. In this installment, I explain why simple reactive data connectors are better than message connectors.

The Problem with Messages

Whenever two or more components cooperate on performing a common task, they must not only be attentive to changes in their own immediate environment, but also, to what the other components are doing. When I first devised the COSA model, I had assumed that sending messages was the best way for components to communicate with one another. I have since changed my mind.

The problem with sending messages is that the message sender has no way of knowing when the message should be sent and, as a result, the receiver has no choice but to query the sender for info. This complicates things because it requires the use of a two-way communication mechanism involving two message connectors or a two-line multi-connector. Alternatively, the sender is forced to send messages all the time whether or not the receiver needs it. I now think that messaging should be used strictly in conjunction with a messenger component dedicated to that purpose. I am also slowly coming to the conclusion that messaging should be used only in a distributed environment between components that reside on separate machines. This means that the COSA pages, in particular, the operating system and software composition pages, are in dire need of a revision. I will expand on this in a future post.

Reactive Data Sensing

The communication method that I prefer is reactive data sensing in which components simply watch one another’s actions within a shared data environment. This way, sharers can immediately sense any change in relevant data and react to it if desired. Since it is up to the receiving components to decide whether or not a change is relevant to them, it makes future extensions easier to implement. Sure, you can have restrictions such as which components have permission to effect changes but the component that owns the data should not impose any restriction on when or whether those changes are detected and used by others. Reactive data sensing is a simple matter of creating sensors (comparison operators) and associating them to relevant effectors. Effector-sensor association is done automatically in COSA.

In the figure above, the dotted line means that the + effector is associated with the != sensor. The way it works is that the != comparison operation is performed automatically every time the effector executes its operation. If the assigned change occurs, the sensor emits a signal.

Control Timing

Reactive sensing makes sense within the context of behavior control. The primary reasons for stopping or deactivating a component are that a) its services are either not needed at certain times (deactivation saves cycles) or b) they would conflict with the work of another component (this prevents errors). The main job of a COSA Supervisor is to precisely time the activation and deactivation of various components under its charge so as to ensure smooth cooperation while eliminating conflicts and improving system performance.

Data Connectors

A data connector is just mechanism for sharing data. The data must reside in only one component, the owner or server. The latter has read/write permission on its own data. A client component that is attached to a data owner via a data connector has only read permission by default. It is up to the component designer to specify whether client components can have write permission as well. The figure below illustrates the use of both data and signal connectors. To the right is the color convention that I currently use for the connectors. Note that control connectors are signal connectors.The data assigned to a data connector can be a single variable, a list of variables, a C++ like data structure or an array. In a design environment, double-clicking on a data connector will open up a box showing the type of data that is assigned to it and the effector and sensors assigned to each data item, if any.

In Part VI, I will introduce a new COSA high-level behavior control mechanism with two complementary commands, ‘Pause’ and ‘Continue’.